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Abstract
The vitreous gel is a transparent extracellular matrix that fills the cavity behind
the lens of the eye and is surrounded by and attached to the retina. This gel liquefies
during ageing and in 25-30% of the oppulation the residual gel structure eventually
collapses away from the posterior retina in a process called posterior retina in a
process called posterior vitreous detachment. This process plays a pivotal role in
a number of common blinding conditions including rhegmatogenous retinal detachment,
proliferative diabetic retinopathy and macular hole formation. In order to understand
the molecular events underlying vitreous liquefaction and posterior vitreous detachment
and to develop new therapies it is important to understand the molecular basis of
normal vitreous gel structure and how this is altered during ageing. It has previously
been established that a dilute dispersion of thin (heterotypic) collagen fibrils is
essential to the gel structure and that age-related vitreous liquefaction is intimately
related to a process whereby these collagen fibrils aggregate. Collagen fibrils have
a natural tendency to aggregate so a key question that has to be addressed is: what
normally maintains the spacing of the collagen fibrils? In mammalian vitreous a network
of hyaluronan normally fills the spaces between these collagen fibrils. This hyaluronan
network can be removed without destroying the gel structure, so the hyaluronan is
not essential for maintaining the spacing of the collagen fibrils although it probably
does increase the mechanical resilience of the gel. The thin heterotypic collagen
fibrils have a coating of non-covalently bound macromolecules which, along with the
surface features of the collagen fibrils themselves, probably play a fundamental role
in maintaining gel stability. They are likely to both maintain the short-range spacing
of vitreous collagen fibrils and to link the fibrils together to form a contiguous
network. A collagen fibril-associated macromolecule that may contribute to the maintenance
of short-range spacing is opticin, a newly discovered extracellular matrix leucine-rich
repeat protein. In addition, surface features of the collagen fibrils such as the
chondroitin sulphate glycosaminoglycan chains of type IX collagen proteoglycan may
also play an important role in maintaining fibril spacing. Furthering our knowledge
of these and other components related to the surface of the heterotypic collagen fibrils
will allow us to make important strides in understanding the macromolecular organisation
of this unique and fascinating tissue. In addition, it will open up new therapeutic
opportunities as it will allow the development of therapeutic reagents that can be
used to modulate vitreous gel structure and thus treat a number of common, potentially
blinding, ocular conditions.